The conventional technique for pumping oil from underground reservoirs utilizes a reciprocating downhole pump. At the surface, a motor drives a walking beam which has connected at one end a polished rod and a string of sucker rods which extend down into the borehole for connection to the pump. As the motor runs, the walking beam raises and lowers the string of sucker rods which causes the pump to lift the fluid from the reservoir up to the surface.
A number of different occurrences can take place at the bottom hole location of the pump. The fluid level may draw down to the level of the pump, the pump can become jammed with sand or the pump can become defective. There also could be other downhole problems. Often, the conditions at the pump can be determined by measuring the load on the sucker rod. It is therefore important in well production to determine the load on the sucker rod to control when the well is pumped or to notify an operator when there is a problem that requires action.
The load on the sucker rod can be measured by use of a load cell which has a strain gauge. One such use of a strain gauge load cell for making a load measurement on a sucker rod is shown in U.S. Pat. No. 4,090,405 to McKee. This patent shows a segmented device which is clamped to the rod and has gripping surfaces above and below the strain gauge for transferring rod load to the device for stressing the strain gauge. U.S. Pat. No. 3,355,938 to Neely et al. describes a load cell which includes a jack. This load cell is mounted on the rod between a hanger bar and rod clamp. A screw mechanism is provided for transferring load to a pair of load cells which include strain gauges. A still further device is a load cell made by Interface Inc. of Scottsdale, Ariz. This is referred to as a LoadTrol (.TM.) load cell. This cell is installed on the polished rod above the hanger bar and below the rod clamp. As this device is compressed, the strain gauge mounted on the device measures the applied load.
A significant problem in the use of rod mounted load cells is the uneven application of the load to the body of the cell. The strain gauges are typically mounted in pairs on opposite sides of the body of the load cell. It has typically been the case that the load has not been uniformly applied to the load cell. For a load cell mounted between a hanger bar and a rod clamp, it is unusual to have a hanger bar which is perfectly level. Likewise, the rod clamps, and any intervening washers, are often not square to the top of the load cell. In addition, the polished rod itself likely is off center from the load cell. Any of these configurations can cause an unequal application of force to the load cell. When this occurs, certain of the strain gauges will be heavily loaded while other of the strain gauges will have reduced or very light loading. Strain gauges must be calibrated against a known load and the operation of the gauges is typically not linear. Thus, if a uniform calibration load is applied to a cell and the output of the strain gauges measured and compared to the load, it is unlikely that a similar size load unequally applied to the cell will produce the same output from the strain gauges. Thus, to obtain accurate information, it is important that the load on the rod be uniformly applied about the load cell.
A further problem of unequal loading is that the lifetime of components such as strain gauges and connecting wires can be substantially reduced if they are overloaded.
Therefore, there exists a need for a load cell which provides even loading across the cell.